Unit 5�Data Converters and PLDs

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Volatile and Non Volatile Memory

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Classification of Memory

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Difference between RAM and ROM

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Difference between Static RAM and Dynamic RAM

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Types of ROM

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Types of DAC

There are two types of DAC

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• Weighted Resistor DAC
• R-2R Ladder DAC

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Weighted Resistor DAC

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Weighted Resistor DAC

1. Let the 3-bit binary input is b2 b1 b0.
2. Here, the bits b2 and b0 denote the Most significant Bit (MSB) and Least Significant Bit (LSB) respectively.
3. The digital switches shown in the above figure will be connected to ground, when the corresponding input bits are equal to ‘0’.
4. Similarly, the digital switches shown in the above figure will be connected to the negative reference voltage, −VR when the corresponding input bits are equal to ‘1’.
5. In the above circuit, the non-inverting input terminal of an op-amp is connected to ground. That means zero volts is applied at the non-inverting input terminal of op-amp.
6. According to the virtual short concept, the voltage at the inverting input terminal of opamp is same as that of the voltage present at its non-inverting input terminal. So, the voltage at the inverting input terminal’s node will be zero volts.

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Weighted Resistor DAC

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Weighted Resistor DAC

The above equation represents the output voltage equation of a 3-bit binary weighted resistor DAC. Since the number of bits are three in the binary (digital) input, we will get seven possible values of output voltage by varying the binary input from 000 to 111 for a fixed reference voltage, VR.

Generalized output voltage equation of an N-bit binary weighted resistor DAC is

The disadvantages of a weighted resistor DAC are as follows −

1. The difference between the resistance values corresponding to LSB & MSB will increase as the number of bits present in the digital input increases.

2. It is difficult to design more accurate resistors.

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R-2R Ladder DAC

The R-2R Ladder DAC overcomes the disadvantages of a binary weighted resistor DAC. As the name suggests, R-2R Ladder DAC produces an analog output, which is almost equal to the digital (binary) input by using a R-2R ladder network in the inverting adder circuit.

The circuit diagram of a 3-bit R-2R Ladder DAC is shown in the following figure −

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R-2R Ladder DAC

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R-2R Ladder DAC

Let the 3-bit binary input is b2 b1 b0. Here, the bits b2 and b0 denote the Most Significant Bit (MSB) and Least Significant Bit (LSB) respectively.

The digital switches shown in the above figure will be connected to ground, when the corresponding input bits are equal to ‘0’. Similarly, the digital switches shown in above figure will be connected to the negative reference voltage, −VR when the corresponding input bits are equal to ‘1’.

The advantages of a R-2R Ladder DAC are as follows −

• R-2R Ladder DAC contains only two values of resistor: R and 2R. So, it is easy to select and design more accurate resistors.
• If more number of bits are present in the digital input, then we have to include required number of R-2R sections additionally.

Due to the above advantages, R-2R Ladder DAC is preferable over binary weighted resistor DAC.

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DAC Specifications

1. Resolution :
• It is defined as the number of different analog output voltage levels that can be provided by a DAC.
• Or alternatively resolution is defined as the ratio of a change in output voltage resulting for a change of 1 LSB at the digital input.
• Simply, resolution is the value of LSB.
2. Accuracy :
• It is the comparison of actual output voltage with expected output
• It is expressed in percentage
• The accuracy of DAC should be, at worst, +- ½ of LSB
3. Settling Time :
• The settling time is the time required for the DAC output to go from 0 to full scale as the binary input changes from all 0s to 1s

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DAC Specifications

4. Linearity :

• The linearity of a D/A converter is defined as the exactness with which the digital input is converted into analog output.
• An ideal D/A converter produces equal increments or step sizes at output for every change in equal increments of binary input.

5. Monotonicity :

• A Digital to Analog converter is said to be monotonic if the analog output increases for an increase in the digital input.

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Difference between Weighted register DAC and R-2R ladder DAC

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Applications of DAC

1. Audio Amplifier - DACs are used to produce DC voltage gain with Microcontroller commands.
2. Video Encoder - The video encoder system will process a video signal and send digital signals to a variety of DACs to produce analog video signals of various formats, along with optimizing of output levels.
3. Display Electronics - The graphic controller will typically use a lookup table to generate data signals sent to a video DAC for analog outputs such as Red, Green, Blue (RGB) signals to drive a display.
4. Data Acquisition Systems - Data to be measured is digitized by an Analog-to-Digital Converter (ADC) and then sent to a processor. The data acquisition will also include a process control end, in which the processor sends feedback data to a DAC for converting to analog signals.
5. Calibration - The DAC provides dynamic calibration for gain and voltage offset for accuracy in test and measurement systems.
6. Motor Control - Many motor controls require voltage control signals, and a DAC is ideal for this application which may be driven by a processor or controller.

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